The purpose of this study is to examine local regulatory mechanisms in the micro-circulation. For this purpose we are measuring the flow in individual capillaries and arterioles with a photometric system which provides an on-line readout of red cell velocity. The response to arterial pressure changes is of particular interest to us. We are doing experiments on cat mesentery and cat sartorius muscle in which square wave pressure pulses are introduced into the arterial perfusion system. We are also studying the patterns of reactive hyperemia in capillaries of various tissues.